The present invention relates to a negatively chargable toner for electrophotography to be charged negatively, which is used for converting an electrostatic latent image into a visible one in a dry electrophotographic process.
Toner particles used for converting an electrostatic latent image into a visible one in a dry electrophotographic process are produced by premixing, in general, a binder, a coloring agent and a charge control agent, with, when necessary, a magnetic powder and some other additives added thereto, and subjecting the mixture to melt kneading, grinding and then classification to obtain particles with a desired particle diameter. In the case where such a toner is used as a two-component developer, the toner particles are mixed with a magnetic powder with stirring. Charge is accumulated on the surfaces of the toner particles due to friction between the toner particles.
On the other hand, when the toner is used as a single-component developer, charge is accumulated on the surfaces of the toner particles due to friction between the toner particles and a sleeve or the like. The charge thus accumulated is utilized for visualizing an electrostatic latent image (development).
In such triboelectric charging, it is necessary that the surfaces of the toner particles be charged either positively or negatively depending upon the type of a photoconductor used for forming thereon an electrostatic latent image and that the amount of the toner charge be made sufficient to more precisely visualize an electrostatic latent image. For this reason, a charge control agent or an electrically conductive material is usually mixed with and dispersed in the binder.
Heretofore, metal-complex-salt-containing dyes as disclosed in Japanese Patent Publication No. 26478/1970 and the like have been generally used as charge control agents so that toner particles can be negatively charged. However, such metal-complex-salt-containing dyes are of complicated structure and have unstable properties. Therefore, they are affected thermally or mechanically during the step of melt kneading or grinding in the production of a toner, and readily undergo decomposition or deterioration. Their charge-controlling properties are thus impaired, and the resulting toner is to have drastically deteriorated properties.
Further, the metal-complex-salt-containing dyes have low compatibility with thermoplastic resins which are used as binders, so that they can exist in the binders only in such a state that their particles are being dispersed in the binders, just like coloring agents. For this reason, the charge control agent existing on the surface layers of the toner particles falls during the step of grinding in the production of the toner, or due to fluid friction caused in a copying machine when charge is accumulated on the surfaces of the toner particles. As a result, the amount of the toner charge is changed, and the toner particles have different amounts of charge. It is thus difficult to stably maintain the toner in a charged state for a long period of time. Furthermore, the metal-complex-salt-containing dyes are, in general, colored, and poor in transparency. Therefore, even if the toner is made into a color toner in a desired color so as to meet the trend toward color duplication, a colored image cannot be clearly obtained by using such a color toner.
In order to improve the aforementioned shortcomings of the metal-complex-salt-containing dyes to be used as the charge control agents, we previously invented a toner for electrophotography to be charged negatively which comprises as a negative charge control agent a copolymer obtained by copolymerizing a polymerizable compound having negative-charge-controlling properties with a polymerizable vinyl monomer copolymerizable therewith, dispersed in a binder, and made an application for patent (Japanese Laid-Open Patent Publications Nos. 184762/1988 and 56974/1991).
Since the above-described copolymer-type negative charge control agent is highly compatible with a synthetic resin which serves as the binder, it does not bring about, unlike the metal-complex-salt-containing dyes, such a problem that the amount of toner charge is changed or diversified due to the falling of the charge control agent. In addition, in order to meet the trend toward color duplication, the toner comprising this charge control agent can be made into a color toner which can clearly produce an image in a desired color. However, since acrylamide methylpropanesulfonic acid is used as the polymerizable compound having negative-charge-controlling properties, the toner properties are impaired at high temperatures and high humidities; in particular, a decrease in the amount of the toner charge is relatively large. Therefore, there is such a problem that it is difficult for the toner to stably produce a clear image over a long period of time.